Geology Reference
In-Depth Information
Fig. 5.16
Photo of the channel in the tightly meandering reach
of the Salmon River, Bay of Fundy (Fig.
5.1a
,
inset
). The gravel
in the channel thalweg was deposited by river floods, whereas
the horizontally bedded sediment on the bank, which consists of
very fine sand, silt and clay with tidal rhythmites, was deposited
by tidal processes
parallel-laminated fine to very fine sand with scarce
mud drapes and limited bioturbation). In deeper chan-
nels that contain coarser sediment, dunes will be pres-
ent, and the deposits there will be cross bedded. In the
outer part of the tidal-fluvial transition, fluid-mud
deposits can be an important component of the chan-
nel-bottom facies (cf. Schrottke et al.
2006
). These
fluid-mud layers can be recognized by the presence of
anomalously thick (i.e. >1 cm before compaction),
structureless to faintly-laminated mud layers that lack
contemporaneous bioturbation (Ichaso and Dalrymple
2009
). The sediment interbedded with the fluid-mud
layers is likely to be the coarsest material that occurs in
that part of the system, producing a markedly bimodal
association of river-flood deposits and tidally depos-
ited fluid muds. This bimodality is likely to be most
pronounced near the bedload convergence area, where
depositional conditions alternate seasonally (Fig.
5.16
).
If dunes are present on the channel floor, the fluid muds
are preferentially preserved in their troughs (Fig.
5.17
;
cf. Schrottke et al.
2006
), generating muddy bottomset
and toeset deposits. The sands in these channel depos-
its will fine upward, whereas the amount of mud and
mud-layer thickness will decrease upward, producing
an upward-cleaning, but upward fining succession
(Dalrymple
2010b
). In channels that lack significant
river input of coarse material, such as the smaller tribu-
tary channels that drain low-lying coastal areas
(Fig.
5.3a
), the channel-bottom deposits can consist
almost entirely of thick fluid-mud layers, with chan-
nel-bank slump deposits and patchy development of
mud-clast breccias.
5.4.2.3
Fringing Facies
The axial deposits described in the two preceding sec-
tions are flanked by a suite of generally fine-grained
deposits, that accumulate in the space been the active
funnel-shaped network of channels and any valley
walls that border the estuary. In narrow, rock-walled
estuaries, the channels can occupy the entire width of
the valley (e.g. Cobequid Bay, Bay of Fundy; Dalrymple
et al.
1990
), whereas broad valleys in soft, coastal-
plain sediments can have wide muddy tidal flats and
marshes (e.g. the South Alligator River, Northern
Australia; Woodroffe et al.
1989
). The nature of these
fringing facies varies with position along the length of
the estuary, and with distance away from the channels
(Dalrymple et al.
1991
).
The margins of the outer part of most estuaries are
erosional, and older material, including mudflat and
salt-marsh deposits that accumulated earlier in the
transgression, can be exposed on the intertidal fore-
shore (cf. Allen
1990
; Cooper et al.
2001
). This ero-
sional surface can be covered by a blanket of mud
during periods of low wave activity (e.g. the summer),
but it is typically removed by winter waves. Bioturbation
